AN EXPERIMENTAL STUDY OF ICE-BED SEPARATION DURING GLACIER SLIDING

Separation of sliding ice from hard beds plays a central role in theories of subglacial hydrology, sediment transport, and quarrying of subglacial bedrock. Despite a half-century of interest in cavities at glacier beds, there are no data establishing relationships among steady cavity size, bed geometry, sliding speed, and effective pressure. Field studies are complicated by unsteady behavior and various poorly-known factors, including the local drag on the bed, bedrock geometry, and cavity size.

The new ring-shear device, ISUSS (Iowa State University Sliding Simulator), allows sliding and ice-bed separation to be studied experimentally. The apparatus drags a ring of ice (0.9 m O.D, 0.2 m wide, 0.2 m thick) across a stepped, rigid bed. The steps are 0.18 m long and 0.027 m high along the ice-ring centerline, with treads inclined uniformly 8° up-flow. Sliding speed and effective pressure are controlled, while cavity volume, bed and wall temperatures, and shear stress are recorded. A glycol-water mixture, which is regulated to ±0.01°C with an external circulator, keeps ice at the melting temperature and melt rates low. Post-experimental measurements of the ice ring's basal topography provide reconstructions of cavity geometries.

Monotonic cavity growth towards a larger, steady size in response to increased sliding speeds was expected. Instead, cavities initially grew past their steady-state volume, followed by a series of progressively damped oscillations above and below steady dimensions before reaching a steady size. Steady-state cavities initiated at step edges and had slightly curved roofs. Using measured cavity geometries, a new model of ice-bed separation based on Nye’s borehole closure theory and mass conservation was tested. The experimental cavity dimensions closely fit modeled geometries, indicating that the new model provides a method for approximating cavity sizes during sliding.